Rivers and Flooding Lab - student handout
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School
Colorado State University, Fort Collins *
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Course
120
Subject
Geology
Date
Dec 6, 2023
Type
docx
Pages
11
Uploaded by ElderMonkey3796
GEOL121 Name _______Jayse W________________
Lab Section ___23_____
Rivers and Flooding Lab
Learning Objectives
After completing this lab, you should be able to:
1)
Design an experiment to learn more about river morphology and behavior.
2)
Calculate the recurrence interval and probability of a given discharge.
3)
Read and interpret a flood plain map to assess flood risk.
Introduction to Rivers and Flooding
Humans have inhabited areas adjacent to rivers for millennia. Traditionally people have settled along
floodplains because of the availability of fertile soils for agricultural purposes. As population increases,
and urban areas grow, development along flood plains changes the way rain and snow infiltrate the
ground, and runoff flows across the surface. Habitation next to rivers also puts people at risk for flood
hazards. Floods are the third deadliest natural hazard in the United States, with only tornados and
lightning strikes ranking higher. In this lab, you will be evaluating river forms and processes, and flood
potential in Fort Collins and on the CSU campus using discharge data to calculate flood recurrence
intervals and examining floodplain maps.
We live in the Cache la Poudre drainage basin, with the Cache la Poudre River flowing from its
headwaters in Rocky Mountain National Park, down Poudre Canyon and through Fort Collins. The
Poudre River joins the South Platte River east of Greeley, Colorado, and the South Platte River joins the
North Platte River in Nebraska, to form the Platte River, which eventually flows into the Missouri River,
and finally the Mississippi River.
Channel planform describes the two-dimensional pattern of the river channel as viewed on a map or an
aerial photograph. The planform of a river reflects sediment available to be transported and stored, and
how the energy of the flowing water interacts with the channel boundaries. Many types of channel
planform exist, but the two most common planforms are meandering and braided rivers
.
The Poudre River is a meandering river through Fort Collins, though it has a straight planform where it
is confined by rock walls in Poudre Canyon. Many boaters and fishermen are interested in the amount of
water flowing within the Poudre River in order to evaluate recreational uses. We talk about the volume
of water moving down a river over a specified time interval as discharge (
Q
) which is calculated using
the Continuity Equation as follows:
Q
=
w
*
d
*
v or Q=vA
,
where w is average width of flow (m), d is average flow depth (m), and v is average velocity (m/s), and A is channel cross sectional area (m
2
). The units of discharge are m
3
/s or ft
3
/s.
To describe the characteristics of floods, we discuss the flood magnitude (discharge or size of the flood,
in ft
3
/s), the duration (how long it lasted), and how common or rare it is (
recurrence interval and
annual probability
). Flood recurrence intervals (
RI
), describe the average number of years between
successive floods of a given size. The recurrence interval is a useful way to describe how often we might
expect a flood of at least a certain size. For example, a 100-year flood is a flood of a given size that
happens once in 100 years on average. The probability of a flood event of a given discharge occurring
during a particular year is called the annual probability
. Annual probability is related to recurrence
interval by the following equation:
P=1/RI
where P = probability (between 0-1), and RI = recurrence interval
A low P or probability (or percentage) indicates that the flood will occur less frequently (high magnitude
floods are more rare) and a high P or probability indicates that it will occur frequently (low magnitude
floods are more common). As you can imagine, this information is important for engineering purposes, as
well as zoning for urban development along rivers. For example, many engineered structures such as
culverts or bridges are typically constructed to handle the 100-year discharge for the river they span. The
probability of the 100-year event occurring during any given year is 1/100 or 0.01 or 1% (low probability
of occurrence). Additionally, recurrence intervals are used to develop floodplain zoning maps in towns to
inform homeowners of flood risks and the need to purchase flood insurance.
Increasing development within urban areas may change the magnitude, duration, and probability of
flooding. As the density of houses and impervious roadways continues to increase in Fort Collins, think
about how this might change the flood hazard. Urbanization played a large role in the destructiveness of
the 1997 Spring Creek flood.
Part 1: Experimenting with River Form and Behavior
In small groups, design an experiment to investigate some aspect of how rivers behave using the stream table. You could investigate changes in channel form, sediment transport, erosion, deposition, etc., which occur as a result of changes in discharge, stream gradient, grain size, etc. Whatever you choose to investigate, you should design your experiment so that you have a clear hypothesis that you are testing and you are only altering one variable at a time. 1) What is the question you are trying to address?
How do different channels affect where water flows during a flood.
2)What is your hypothesis?
If the channel is smaller, it will cause a faster discharge.
3)
List 3 observations from your experiment:
Skinnier channels add depth to the river, larger channels flow slower, and smaller rocks are easily moved by the river while bigger ones stay put.
4) List 3 conclusions from your experiment:
Smaller channels lead to faster discharge rates. Due to faster discharge, there is more movement of
rocks rather than large, more calm channels. If your using a large channel you need to keep the size
because if it gets skinnier there will be flooding.
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Part 2: Discharge Variability and Flooding Recurrence Intervals
Variability in the discharge of a river through time contributes to the form and characteristics of that river.
Flooding constitutes increases in discharge beyond what the banks of a river can handle. The figure below
shows daily discharge measurements in 2021 for the Cache la Poudre River gaging station in Old Town.
The gaging station is operated by the US Geological Survey, and can be accessed via the web at:
http://waterdata.usgs.gov/nwis/uv?06752260
1)
Describe the changes in discharge of the Poudre River through the year. When was discharge the highest? Lowest? What factors might contribute to the variability you observe?
The discharge was highest during May, June, and July, which could be due to the amount of snow that is melting and coming down from the mountains. It gets to its lowest discharge right before the winter season starts, around September and October.
The next figure shows daily discharge measurements for the last ~44 years. You will notice that the
summer peak discharge varies greatly from year to year. We can use the peak discharge from each year to
calculate the recurrence interval and probability of a given discharge.
The data on the next page are a subset of 40 years of data from a USGS gauging station on the Poudre
River. On the data table provided, the discharges are ranked in descending order, giving the highest
discharge a ranking of 1, and the second highest a ranking of 2, etc.
2)
Calculate the recurrence interval for the five blank years using the equation:
RI= (n+1)
m
where n = total number of years of data, in this case n=40,
m = rank number of specific discharge
Date
Annual Peak Dis- charge (Q)
(ft
3
/s)
Rank (m) (years)
Recurrence Interval (RI)
6/18/95
3500
8
5.13
6/16/96
2670
18
2.28
6/9/97
3290
12
3.42
6/4/98
959
33
1.24
4/30/99
7710
2
20.50
5/6/00
785
36
1.14
5/30/01
713
40
1.03
5/31/02
751
38
1.08
5/30/03
2100
21
1.95
7/1/04
997
31
1.32
6/4/05
1720
26
1.58
10/30/05
1210
29
1.41
6/13/07
933
34
1.21
6/7/08
1830
23
1.78
6/22/09
1790
25
1.64
6/12/10
4570
6
6.83
6/9/11
3220
13
3.15
7/30/12
764
37
1.11
9/13/13
8140
1
41
5/31/14
5860
4
10.25
3)
Plot the RI vs. the Discharge values from the table above on the semi-log graph paper provided on the next page. Logarithms are used to show several orders of magnitude of data. Draw a best-fit line to your data.
Here is a resource explaining how to plot points: https://serc.carleton.edu/mathyouneed/graphing/plotting.html
And this page describes how to draw a best fit line: https://serc.carleton.edu/mathyouneed/graphing/bestfit.html
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4) What is the relationship between recurrence interval (RI) and discharge (Q)?
It’s a linear relationship
5) The Fall 2013 Front Range flooding caused devastation along much of the Front Range. What is the probability (P) of a flood the same size as the 2013 flood on the Poudre River happening again next year?
(Remember P = 1/RI)
P=1/41=0.02
6) Use the graph to estimate the discharge associated with the 100-year flood on the Poudre
River. You will need to extrapolate a best-fit line. How does this discharge compare to that of
Fall 2013?
The 100-year flood will have a much greater discharge than the flood in the fall of 2013.
Part 3: Examining Flood Maps
Obtain the Fort Collins Flood Risk Map
. You can also view a GIS version of this map at: https://
www.fcgov.com/utilities/what-we-do/stormwater/flooding/floodplain-maps-documents
1) What are the flood risk categories shown on the map legend? What are the flood recurrence intervals
associated with each flood risk designation?
It shows what areas are in either high or moderate risk of floods for the floodway, 100-year, and 500-year floods.
2) Reflecting on the RI calculations and discharge data in Part 1, why do you think the 500- year
floodplain is wider than the other floodplains?
The 500-year flood is going to much larger so they are preparing for it with a larger floodplain.
3) Find W. Elizabeth St. on the map, just west of Shields. Why do you think Elizabeth Street is marked
as a flood zone on the map? It sitting at a low elevation and has a couple streams that could easily flood leading right into it.
4) What happens when the water from W. Elizabeth enters campus? Does it get blocked like the map
may suggest? No, the water will keep moving through campus at it has done before, leaving water damage at Morgan
library and surrounding areas.
5) Examine the CSU Floodplain Map. Where are the most flood-prone places on campus? Are they the
areas you would have expected? Why or why not?
The parking lot near Morgan library was more flood-prone than I had realized. This makes sense,
though, due to the IM fields being a direct path right to it.
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6) Locate your place of residence on the floodplain map. (If you don’t live on the map, pick a location you
go to frequently.) Do you live in a flood zone? If so, what is the risk? If you live on campus, do think that your dorm is susceptible to flood waters? In which RI floodplain do you live?
I believe I’m in a horrible flood zone due to being right on Laurel but then also having North Drive right behind me; I’m sandwiched between two large flood zones. I’m near the Laurel Street Basin but I technically don’t live in a flood plain.
7) Locate the intersection of N. Lemay and E.Vine Dr. If you were to buy property here, would it be in a floodplain? If so, do you think flood insurance is required? What are some of the risks you may assume as
a homeowner in a floodplain?
Yes, I would be living in a floodplain but I don’t think flood insurance is a requirement but I would buy it.
8) If you lived on a floodplain with a 500 year RI, would you buy flood insurance? What is the probability that the flood would occur in a given year, and damage your house? Based on this probability, if a flood did occur while you lived there, would this be an acceptable risk to you?
I would accept the flood risk and not pay the insurance since there is a 1 in 500 chance I experience a flood of this caliber.
9) The flood recurrence intervals that you see on these maps are determined using data from stream gauges, just as you did in Part 2. Are there any reasons you can think of why the likelihood of floods in the future may be different than the likelihood determined from past data?
They learned from the last flood and have changed some infrastructuree as well as adding flood measures in case of another one.
10) What is still unclear about calculating flood recurrence intervals or interpreting flood maps? What questions do you have?
Who would I contact for flood insurance if I buy a home in a at risk area?